Ab initio description of the fragmentation of H2O+( 2B2)

The Journal of Physical Chemistry A, 1999

We develop a wave packet approach to treating the electronically nonadiabatic reaction dynamics of O( 1 D) + H 2 f OH + H, allowing for the 1 1 A′ and 2 1 A′ potential energy surfaces and couplings, as well as the three internal nuclear coordinates. Two different systems of coupled potential energy surfaces are considered, a semiempirical diatomics-in-molecules (DIM) system due to Kuntz, Niefer, and Sloan, and a recently developed ab initio system due to Dobbyn and Knowles (DK). Nonadiabatic quantum results, with total angular momentum J ) 0, are obtained and discussed. Several single surface calculations are carried out for comparison with the nonadiabatic results. Comparisons with trajectory surface hopping (TSH) calculations, and with approximate quantum calculations, are also included. The electrostatic coupling produces strong interactions between the 1 1 A′ and 2 1 A′ states at short range (where these states have a conical intersection) and weak but, interestingly, nonnegligible interactions between these states at longer range. Our wave packet results show that if the initial state is chosen to be effectively the 1A′ state (for which insertion to form products occurs on the adiabatic surface), then there is very little difference between the adiabatic and coupled surface results. In either case the reaction probability is a relatively flat function of energy, except for resonant oscillations. However, the 2A′ reaction, dynamics (which involves a collinear transition state) is strongly perturbed by nonadiabatic effects in two distinct ways. At energies above the transition state barrier, the diabatic limit is dominant, and the 2A′ reaction probability is similar to that for 1A′′, which has no coupling with the other surfaces. At energies below the barrier, we find a significant component of the reaction probability from long range electronic coupling that effectively allows the wave packet to avoid having to tunnel through the barrier. This effect, which is observed on both the DIM and DK surfaces, is estimated to cause a 10% contribution to the room temperature rate constant from nonadiabatic effects. Similar results are obtained from the TSH and approximate quantum calculations.

Journal of Physics B: Atomic, Molecular and Optical Physics, 2005

We investigate fast, highly charged ion-induced HOD molecule fragmentation dynamics. Focusing on double ionization of the molecule, we evidence a strong preferential cleavage of the O-H bond rather than the O-D bond. We find an isotopic branching ratio defined as the number of H + + OD + over D + + OH + dissociations of 6.5 ± 0.5. Moreover, the coincident measurement, by imaging techniques, of high-resolution kinetic energy release (KER) distributions for different fragmentation channels shows, for the first time, a clear difference of approximately 1 eV between the mean KER value of the two-body dication fragmentation channels H + + OD + and D + + OH + . We analyse the two aspects of this isotopic effect by means of a semi-classical calculation simulating the dissociation dynamics via the dication electronic ground state. The final pathway is found to be strongly correlated to the initial position and momentum distributions. Those correlations explain both the observed O-H bond preferential cleavage and, for a large part, the different mean KER values. The other part of the 1 eV kinetic energy difference results from different vibrational energies of the residual molecular fragment that have also been estimated.

The Journal of Physical Chemistry A, 2001

Rotational and vibrational state distributions have been measured for the OH/OD(A 2 Σ + ) fragment produced by the photodissociation of H 2 O/D 2 O at a set of wavelengths in the 133-119 nm range that lead to excitation of the B ( 1 A 1 ) state, either directly or through particular absorption features of the C 1 B 1 r X 1 A 1 and D 1 A 1 r X 1 A 1 Rydberg transitions. The experimental distributions are obtained from the A 2 Σ + -X 2 Π fluorescence spectra using a truncated single value decomposition method (TSVD). They are compared to distributions calculated in a complete three-dimensional quantum mechanical treatment by using two sets of diabatic potential energy surfaces obtained by Dobbyn and Knowles (DK) and by van Harrevelt and van Hemert (Leiden). The measured vibrational branching ratios seem to be better accounted for by the former and the rotational distributions by the latter. However, a more detailed analysis indicates that the DK surface would presumably account for the experiment if the room-temperature rotational distribution of the parent water molecule were introduced in the calculation. The experimental results are therefore interpreted as a confirmation of the more direct and shorter-lived trajectories favored by the DK surface.

Physical Chemistry Chemical Physics, 2013

The angular anisotropy of fragments created in the dissociation of core-electron excited water molecules is studied to probe the correlation between fragmentation channels, kinematics and molecular geometry. We present fragment kinetic measurements for water molecules where the innershell oxygen electron is excited to the unoccupied 4a 1 and 2b 2 valence molecular orbitals. The kinematics of individual fragmentation channels are measured using fully three-dimensional momentum imaging of fragments. The results show that the geometry of the molecule and the kinetic energy of fragments are strongly coupled in the atomisation process. In addition we identify a fragmentation process arising from bond rearrangement evidenced by the H 2 + -O + ion pair which is accessible for resonant excitation of the 1s electron. In all of the two-body fragmentation processes the dissociation takes place along the potential-energy surface, while atomisation reveals both dissociation along the potential surface and Coulomb explosion. The angular distribution of fragments suggests that the bond rearrangement is very rapid; likely on a sub 10 fs time scale.

Chem Phys Lett, 2001

An ab initio QCISD/6-31+G ∗ direct molecular dynamics (MD) study of the reaction of C + with water has been performed and the mechanism of reaction examined. Principal products are isoformyl cation, HOC +, and H atom. There are two main channels, direct `knock out' of a hydrogen, and via an intermediate, H 2OC +, The intermediate is one not previously proposed in either experimental or theoretical studies of the reaction. Most product HOC + molecules were found to be internally energetic enough to isomerize to formyl cation, HCO +.

The workability of beyond Born-Oppenheimer theory to construct diabatic potential energy surfaces (PESs) of a charge transfer atom-diatom collision process has been explored by performing scattering calculations to extract accurate integral cross sections (ICSs) and rate constants for comparison with most recent experimental quantities. We calculate non-adiabatic coupling terms among the lowest three singlet states of H + 3 system (1 1 A , 2 1 A , and 3 1 A ) using MRCI level of calculation and solve the adiabatic-diabatic transformation equation to formulate the diabatic Hamiltonian matrix of the same process [S. Mukherjee et al., J. Chem. Phys. 141, 204306 (2014)] for the entire region of nuclear configuration space. The nonadiabatic effects in the D + + H 2 reaction has been studied by implementing the coupled 3D time-dependent wave packet formalism in hyperspherical coordinates [S. Adhikari and A. J. C. Varandas, Comput. Phys. Commun. 184, 270 ] with zero and non-zero total angular momentum (J) on such newly constructed accurate (ab initio) diabatic PESs of H + 3 . We have depicted the convergence profiles of reaction probabilities for the reactive non-charge transfer, non-reactive charge transfer, and reactive charge transfer processes for different collisional energies with respect to the helicity (K) and total angular momentum (J) quantum numbers. Finally, total and state-to-state ICSs are calculated as a function of collision energy for the initial rovibrational state (v = 0, j = 0) of the H 2 molecule, and consequently, those quantities are compared with previous theoretical and experimental results. Published by AIP Publishing. [http://dx.

Physical Review Letters, 2010

Photofragmentation of the protonated water dimer H þ ðH 2 OÞ 2 , a fundamental system both in aqueous solutions and gas-phase water clusters, has been studied at 13.8 nm using the Free Electron Laser FLASH in Hamburg. In a crossed-beam experiment using time-resolved, single-molecule fragment imaging, the two-body breakup into H 2 O þ þ H 3 O þ was found as a prominent fragmentation channel with a kinetic energy release of up to 10 eV. This channel was observed with at least a similar yield as events with stronger fragmentation, producing protons together with neutral fragments and showing an absolute cross section of ð0:5 AE 0:2Þ Â 10 À18 cm 2 .

Physical Review A, 2007

Charge exchange and fragmentation in the collision systems He 2+ +H 2 O and He + +H 2 O are theoretically investigated at projectile energies of a few keV. The calculations are based on the electron nuclear dynamics ͑END͒ method which solves the time-dependent Schrödinger equation. Total and differential cross sections for charge exchange are evaluated by averaging over 10 orientations of the H 2 O molecule. Summed total electron capture cross sections are found to be in good agreement with available experimental data. Projectile scattering was studied in the full angular range with respect to the incident beam direction. The theory provides a description of the fragmentation mechanisms such as Coulomb explosion and binary collision processes. For impact parameters below 3.5 a.u., we find that single and double electron capture occurs, resulting always in full fragmentation of H 2 O independent of the target orientation for 3 He 2+ ions. Hydrogen and oxygen fragments and its respective ions, are studied as a function of emission angle and energy. In the binary collisions regime the theoretical results are found to be in excellent agreement with previous experimental data. In the Coulomb explosion regime the theoretical data are found to peak at specific angles including 90°, which is consistent with the experiment.

The Journal of Chemical Physics, 2004

Tunneling splittings in the water dimer have been determined by the semiclassical WKB method, based on pathways characterized at the computational level of second-order Møller-Plesset ͑MP2͒ theory with basis sets of aug-cc-pVTZ quality. This calculation takes into account all three acceptor tunneling, donor-acceptor interchange, and bifurcation tunneling rearrangements of the water dimer. The tunneling splittings were evaluated as 7.73 cm Ϫ1 ͑large splitting͒ and 0.42 cm Ϫ1 ͑small splitting͒, which are in good agreement with the corresponding experimental values of 11.18 cm Ϫ1 and 0.70 cm Ϫ1 , respectively.

Ultrasonics Sonochemistry, 2019

Quantum tunneling in chemistry is often attributed to the processes at low or near room temperatures when the rate of thermal reactions becomes far less than the rate of quantum tunneling. However, in some rapid processes, quantum tunneling can be observed even at high temperatures. Herein, we report the experimental evidence for anomalous H/D kinetic isotope effect (KIE) during sonochemical dissociation of water molecule driven by 20 kHz power ultrasound measured in H 2 O/D 2 O mixtures saturated with Ar or Xe. Hydrogen released during ultrasonic treatment is enriched by light isotope. The observed H/D KIE (α=2.15-1.50) is much larger than what is calculated assuming a classical KIE for T g =5000 K (α=1.15) obtained from the sonoluminescence spectra in H 2 O and D 2 O. Furthermore, the α values sharply decrease with increasing of H 2 O content in H 2 O/D 2 O mixtures reaching a steady-state value close to α=1.50, which also cannot be explained by O-H/O-D zero-point energy difference. We suggest that these results can be understood in terms of quantum electron tunneling occurring in nonequilibrium picosecond plasma produced at the last stage of cavitation bubble collapse. Thermal homolytic splitting of water molecule is inhibited by extremely short lifetime of such plasma. On the contrary, immensely short traversal time for electron tunneling in water allows H 2 O dissociation by quantum tunneling mechanism.